Generally, this kind of gear crank apparatus is mounted on a bottom bracket at the bicycle and the driving chain is stretched across the chain gear of the apparatus and the rear chain gear at the rear wheel hub, thereby constituting a drive mechanism. Pedals provided at the crank arms are pedalled to transmit a torque caused by motion of a cyclist's legs to the rear chain gear through the driving chain to thereby drive the rear wheel for the bicycle's running.
The cyclist rides on a saddle at the bicycle and treads the pedals around the crank shaft, in which the motion of his legs is similar to the movement of pendulum while walking, in other words, the motion of pedal has an inherent cycle period and a speed pattern on the basis of length of the fulcrum and the centroid.
The torque output by the cyclist's leg when pedalling, as shown in the pedalling cycle period in FIG. 3, varies in the positions where the crank arms are in proximity to the upper and lower dead points thereof and in a region therebetween, thereby being minimum near both the dead points and maximum after rotation at an angle of about 70.degree..+-.5.degree. ahead of the upper dead point in the driven-rotation direction of each crank arm.
This variation of torque depends on the motion of a cyclist's leg during pedalling and the position of each pedal relative to its motion, which tends to be indifferent regardless of the size and type of bicycle and different muscular strength of the cyclist.
Since an elliptic chain gear has a changeable pitch diameter of each tooth so that a gear ratio of the same to the chain gear of a fixed pitch diameter is changeable, it is deemed that in a case of using the elliptic chain gear as the front chain gear for the bicycle, the maximum gear ratio is set in the region between the upper and lower dead points for obtaining the maximum torque for pedalling, while, the minimum gear ratio at the upper and lower dead points for the minimum torque only.
However, the circumferential speed of the pedal reduces in the region for the maximum gear ratio to thereby lower the power and increases at the upper and lower dead points for the minimum gear ratio to thereby rather apply a physical load on the cyclist.
In other words, in consideration of the motion of the leg, the muscular strength is output to a maximum with less energy consumption in the region between the upper and lower dead points of the crank arms, whereby the increased circumferential speed can improve the power from the relation: Power=Torque.times.Speed. Neverthless, the aforesaid construction reduces the speed in this region to conversely lower the power. On the other hand, at the upper and lower dead points and in proximity thereto, the energy consumption for pedalling is required more than in the above region, but the muscular strength cannot inherently be output, whereby the reduced circumferential speed at both the dead points can reduce the physical load on the muscles of cyclist. Nevertheless, the circumferential speed increases to apply a more physical load onto him.